1. Field of the Invention
The present invention relates to a central-office termination apparatus capable of adaptively changing connection to a subscriber-terminal terminator and a path switching method of changing connection between a subscriber-terminal terminator and central-office termination apparatuses.
2. Description of the Background Art
Recently, in order to provide high-speed broadband service to private premises, services, called an FTTH (Fiber To The Home), using optical fiber transmission lines in transmission channels has been developed. In the provision of the broadband services using the FTTH, an optical access network called a passive optical network (PON) for subscriber terminals is frequently applied.
The PON is configured so that a single central-office termination apparatus, e.g. an optical line terminal (OLT) and a plurality of subscriber-terminal terminators, e.g. optical network units (ONUs) are connected by means of a single optical cable split by an optical passive device called an optical splitter or coupler. In the PON, the optical fiber or OLT is commonly used by a plurality of subscriber terminals to economically provide the FTTH services.
As a sort of PON, there is so-called 10-Gigabit Ethernet (Trademark) PON (10G-EPON) as referred to in IEEE (Institute of Electrical and Electronics Engineers), Std 802.3av-2009. In that sort of PON, a Time Division Multiple Access (TDMA) technique is applied to communication from ONUs to an OLT, i.e. upstream communication, in order to avoid collision of signals transmitted from the ONUs with each other. The PON using the TDMA technique is often called a TDM-PON.
In response to prospective increase of communication demand in optical access networks, research and development are conducted on a WDM (Wavelength Division Multiplexing)/TDM-PON as one of the next generation PONs using transmission rate exceeding 10 Gbps. The WDM/TDM-PON constructs a plurality of TDM-PONs on a single PON infrastructure by the WDM technique, as refer to Japanese patent laid-open publication No. 2011-55407. By means of the TWDM-PON, the transmission capacity of the PON infrastructure can be increased.
The OLT in the TWDM-PON disclosed in Japanese patent laid-open publication No. 2011-55407 is provided with a plurality of optical transmitter/receivers and a controller for controlling the TWDM-PON. The optical transmitter/receivers are connected to a plurality of ONUs via an optical coupler.
In the upstream communication, receiving wavelengths are fixedly allocated to the optical transmitter/receivers of the OLT so as not to overlap with each other between the optical transmitter/receivers. In that solution, the transmission wavelengths of optical transmitter/receivers of the ONU are adapted changeable, thereby adaptively switching connections between the optical transmitter/receivers of the OLT and the ONU. In a downstream communication from the OLT to the ONU, likewise the upstream communication, the transmission wavelengths are fixedly allocated to the optical transmitter/receivers of the OLT, and the reception wavelengths of the optical transmitter/receivers of the ONU are adapted changeable, thereby adaptively switching connections between the optical transmitter/receivers of the OLT and the ONU. Thus, the TWDM-PON is advantageous in load distribution responsive to traffic fluctuation, higher reliability by path switching diversity during failure and power saving by sleep of the optical transmitter/receivers and device circuitry during low load.
In the TWDM-PON, when the connections of the OLT to the ONUs are adaptively switched in the downstream communication, the switching of the optical transmitter/receivers of the OLT and the switching of the reception wavelengths of the ONU are carried out. During a switching period of time until the reception wavelength of the ONU is switched to a new wavelength, the ONU cannot receive a packet of the downstream communication, i.e. downstream packet. However, in a multimedia application, it is preferable that packet loss would not occur during the switching period of time, and thus uninterrupted switching process is required.
Therefore, in order to avoid packet loss in the downstream communication during the switching period of time, it is necessary that the ONUs are adapted for buffering packets meant for that ONU currently under switching.
In a solution proposed for buffering inputted packets and switching a communication path, a buffer is arranged at the stage in front of a switch for switching a path and the switch switches the path in accordance with the destinations of inputted packets, refer to Japanese patent laid-open publication No. 229404/1998.
In the TWDM-PON, the OLT identifies downstream packets in terms of respective ONUs, and then, distributes the packets to optical transmitter/receivers to which the transmission wavelengths corresponding to the reception wavelengths of the respective ONUs are allocated.
When the configuration disclosed in Japanese patent laid-open publication No. 229404/1998 is applied to carrying out uninterrupted path switching in the TWDM-PON, it would be necessary to provide the buffers at the stage in front of the switch correspondingly in number to the ONUs contained in the TWDM-PON. Therefore, if a lot of ONUs are contained in the system, circuit scale is increased. Since the buffer needs its capacity sufficient for storing packets receivable during the switching period of time, if the time for the switching is lengthened, it would be necessary to increase the buffer capacity. The increase of the circuit scale or the buffer capacity would cause a problem in implementing the system.
By contrast, in a system to which a common buffer scheme commonly using a buffer is applied, as the number of the contained ONUs is increased, information for managing addresses of the buffer would be increased. Therefore, if a lot of ONUs are contained in the system, a memory with large capacity is required specifically for managing the addresses, thus also causing a problem in implementing the system.